1. Field of the Invention
This invention relates generally to electrochemical cells and more particularly to a new and improved separator for such cells and of fabric material. This fabric separator is particularly useful in an electrochemical cell having an anode comprising a Group IA, IIA and IIIB metal and a depolarizer/catholyte or a solid cathode/electrolyte system.
Fundamental requirements of a separator for use in an electrochemical cell is that the material of construction be resistant to degradation in the cell environment, have sufficient thickness to maintain interelectrode separation without interfering with cell high performance, and exhibit sufficient surface energy such that electrolyte wettability and absorption are augmented. The separator material must also have a relatively high electrical resistivity in order to prohibit the establishment of short circuit currents flowing directly between the electrodes, through the separator. These requirements are balanced by the need for the separator to have sufficient porosity such that electrode separation is maintained while allowing ionic transfer within the electrolyte to occur unimpeded during intended cell discharge. Additionally, the separator must have sufficiently strong tensile properties to facilitate cell fabrication and to further withstand internal cell stresses due to changes in electrode volume during discharge and re-charging cycles in secondary electrochemical cells.
2. Prior Art
Conventionally, separators have fallen into two general categories - those made of microporous films and those comprising a non-woven fabric made from glass fibers and polymeric fibers. The former type of separator material is shown in Schlaikjer U.S. Pat. No. 4,629,666, which discloses partially halogenated microporous polymeric films for use as a separator in electrochemical cells containing alkali metals, such as lithium, and inorganic electrolytes. Similarly, a microporous film separator comprising polytetrafluoroethylene (PTFE) for use in an electrochemical cell is also disclosed in Strier U.S. Pat. No. 3,661,645 et al. The PTFE film is provided with particles of a substance that is insoluble in water, but which can be leached from the film to produce uniformly distributed pores therein. The benefit is that microporous films can be made very thin which contributes to volumetric efficiency in that the separator does not detract appreciably from the volume of cathode and anode active material and therefore the energy density. The problem is that the reduction in separator thickness is accompanied by a reduction in material strength as microporous films are inherently weak. Rupture of the separator during the manufacturing process is not uncommon and can lead to contact between the electrode materials, thereby resulting in an internal short circuit condition.
The latter, non-woven fabric type of separator is shown in Cieslak U.S. Pat. No. 5,002,843 et al., which discloses a lithium/thionyl chloride electrochemical cell system having a separator made of aramid fibers provided in a non-woven mat form. Although aramid fibers are highly porous and, therefore, not a detriment to ionic transfer within the depolarizer/catholyte, non-woven fabrics are easily torn. In a similar manner as microporous films, the use of non-woven fabric as a separator in an electrochemical cell can result in direct physical contact between the electrodes which would give rise to an internal short circuit condition.